1. Field of the Invention
The present invention relates to a multistage wire drawing apparatus wherein a plurality of wire drawing units are arranged in tandem to draw a high-melting metal, as a work, in a multistage manner.
2. Description of the Prior Art
When wire is manufactured from a high-melting metal such as tungsten or molybdenum, an ingot is prepared by powder metallurgy, and the ingot (work) is drawn by a unit to produce wire. At an early stage of manufacture, the ingot is considerably fragile and tends to break. For this reason, multistage wire drawing to sequentially decrease the wire diameter of the ingot is conventionally performed. In this case, the ingot must be heated by a heating means at each stage of manufacture, and an excessive tension must not act on the ingot.
A wire drawing unit termed a draw bench is used as a machine for drawing wire of a high-melting metal such as tungsten or molybdenum. In the draw bench, a work is inserted in a die, chucked and drawn by a movable chuck.
Conventionally, multistage wire drawing for sequentially decreasing the diameter of the work is performed by a wire drawing unit of the type described above and in the following manner. The work (ingot) is heated, inserted in and drawn by the die of a single wire drawing unit, and then wound by a winder. After the wire is wound, it is drawn from the winder and heated, and then inserted in and drawn by a die of another drawing unit having an aperture diameter different from that of the first die. Wire drawing is repeated in this manner. Wire drawing must be intermittently and independently performed a plurality of times in order to obtain a final wire diameter, resulting in a protracted and inefficient manufacturing process.
In order to overcome this drawback, a multistage wire drawing apparatus is proposed wherein a plurality of wire drawing units, each having a die of a different aperture diameter, are arranged in tandem, and a work is continuously fed and sequentially inserted in the die of each unit to thereby produce the required wire diameter through a single wire drawing.
In the multistage wire drawing apparatus of the above-mentioned type, a work must be moved between front and rear stage wire drawing units under constant tension so that any excessive tension does not act on the work, for the purpose of preventing damage to the work while it is taut. For this purpose, it is proposed to set the movable chuck speed (wire drawing speed) of the front stage wire drawing unit and the movable chuck speed (wire drawing speed) of the rear stage wire drawing unit in accordance with the ratio of the aperture diameters of the dies used in the front and rear stage wire drawing units.
Assuming that the aperture diameter of each die is constant, the ratio of the movable chuck speeds need not be changed once it is set. However, in practice, the aperture diameter of a die may increase due to friction between the aperture portion and the work, or, conversely, may decrease due to expansion of the die as a result of thermal conduction or friction from the heated work. Thus, the die aperture diameter is not constant but changes in practice. As a result, if the movable chuck speed of the rear wire drawing unit is set to be constant irrespective of change in the aperture diameter of each die, the chuck speed may be too fast or too slow in relation to the aperture diameter of the die. Then, the tension of the work between the wire drawing units is not constant but changes, resulting in damage or loosening of the wire, and thereby rendering wire drawing difficult.
It is conventionally difficult to adopt a multistage wire drawing apparatus due to the drawbacks described above.